The present invention relates to binder formulations for use in agglomerating mineral concentrates or fines comprised of colloidal silica and a polymeric binder. More particularly, it is directed to a mineral pellet including the binder formulation and related low temperature hardening process for making the same.
Pelletizing is the most desirable agglomerating process for iron ore. The concentrates produced are of an extremely fine size (85%xe2x80x9444 micrometers) and are readily formed into green pellets.
The process encompasses two basic steps: (1) the formation of sized (xe2x88x92xc2xd+⅝-inch) green (wet) pellets from a moist filter cake concentrate via the balling process, and (2) the oxidation and induration of the green pellet by high temperature heat treatment in an oxidizing atmosphere to produce a fired pellet with sufficient strength and abrasion resistance to withstand the rigors of handling, transportation, storage and blast furnace reduction/smelting.
Different types of additives, which can be classified as binders, fluxes, and fluxing binders are sometimes used to aid in pellet forming, induration and blast furnace reduction. Bentonite, composed mostly of the clay mineral, montomorillonite, is the binder most commonly used to minimize degradation of the green and dry pellet during the induration process. Recently, several water soluble organic binders have been used, in lieu of bentonite, to reduce silica contamination and improve the reducibility of the fired pellet. These organic binders, which include Carboxy Methyl Cellulose (CMC), Alcotac-Acrylate/Acrylamide copolymer and modified starch, are used to eliminate the additional 0.5% silica that bentonite adds to the pellet and also to improve the xe2x80x98reducibilityxe2x80x99 of the fired pellet. A lower xe2x80x98silicaxe2x80x99 pellet reduces the slag volume in the blast furnace and a higher xe2x80x98reducibilityxe2x80x99 pellet increases blast furnace productivity and lowers the coke rate requirement.
Most of the iron ore concentrates produced in North America contain over 95% magnetite. During the heat treating process the magnetite (Fe3O4) is oxidized to hematite (Fe2O3) according to the reaction:
4Fe3O4+O2xe2x86x926Fe2O3
This reaction begins at about 1600xc2x0 F. and is completed around 2000xc2x0 F. if sufficient oxygen is available. The reaction is exothermic and releases about 210 Btus per pound of magnetite. The heat generated provides over half the total heat required for the process. The oxide bonding produced by the conversion of the magnetite to hematite, however, is not sufficient to produce a competent pellet and the pellet must be further heated to a temperature of 1288 to 1343xc2x0 C. (2350-2450xc2x0 F.) to complete the sintering of the hematite grains and slag bonding of the gangue constituents. The final pellet has a compressive strength of over 500 pounds.
The fuel (natural gas) and electric power requirements for producing pellets from magnetic taconite concentrates is currently about 350,000 Btus and 35 KWH per long ton pellets, respectively. The pelletizing process for hematite concentrate is similar to that for magnetite but because there is no exothermic heat release the total fuel requirements are considerably higher.
The mechanical specifications for good shipping xe2x80x98firedxe2x80x99 pellets consist of the following:
(1) Size structurexe2x80x94Pellets should be closely sized, preferably in the xe2x88x92xc2xd+xe2x85x9c-inch size range with less than 2% finer than xc2xc-inch.
(2) Weatherabilityxe2x80x94Pellets must have excellent resistance to long term stock piling and outdoor winter storage. Maximum moisture content should not exceed 3 percent and freezing must not be a problem.
(3) Resistance to Breakage during Handling and Shippingxe2x80x94Pellets must be strong enough to withstand (without substantial breakage) normal handling between the pellet plant (mine site) and the blast furnace. Two standard ASTM tests are available to predict the pellet strength performance. They are the tumble test and the pellet compressive test.
In the tumble test, a 25 pound sample of plus xc2xc-inch pellets are tumbled in a 3-foot diameter by 18-inch wide steel drum (fitted with lifters) for 200 revolutions at a speed of 24 RPM. After tumbling, the pellets are screened at xc2xc-inch and 28 mesh. The weight percent plus xc2xc-inch is referred to as the xe2x80x98tumble indexxe2x80x99 and the percentage of minus 28-mesh fines produced as the xe2x80x98abrasion (dust) indexxe2x80x99. Fired pellets normally have a tumble index greater than 95 percent and an abrasion index of less than 3.5 percent.
In the compressive test, the compressive strength of 60 individual pellets is determined at room temperature with an automatic compressive tester using a constant speed load. The average compressive strength of the xe2x88x92xc2xd+xe2x85x9c-inch pellets should exceed 450 pounds. The percentage of pellets that have a compressive strength of less than 200 pounds is also important since most of the weaker pellets tend to break up during handling, transportation and blast furnace reduction.
Iron ore pellets containing 4-5% silica are used in North America, primarily as a feed stock for blast furnace reduction and smelting. The blast furnace is a counter current furnace which has the ability to reduce and melt burdens and use coke as the source of heat and reducing gases.
In the upper part of the shaft, sometimes referred to as the Massive Zone, the hematite pellets are slowly heated and reduced while descending. Beginning at a temperature of about 450-500xc2x0 C. the hematite in the pellet is reduced to magnetite according to the reaction:
3Fe2O3 (hematite)+COxe2x86x922Fe3O4 (magnetite)+CO2
The reduction of hematite-to-magnetite results in a change in crystal structure that sets up stresses in the pellet that are strong enough to cause significant pellet degradation. The fine particles produced can be either carried out of the furnace in the gas stream or fill the interstices of the burden and reduce its permeability. The performance of pellets in this section of the furnace can be predicted with the standard low temperature breakage (LTB) test.
At intermediate levels in the Massive Zone, the pellets begin to increase in temperature, i.e. 500-1000xc2x0 C. and the magnetite begins to reduce to wustite and wustite to metallic iron according to the reactions:
Fe3O4 (magnetite)+COxe2x86x923FexO (wustite)+CO2
FexO (Wustite)+COxe2x86x92Fe (metallic)+CO2
The relative rate of reduction is the critical test parameter in this section of the Massive Zone. It is measured by a test procedure known as the ISO-reducibility, xe2x80x94i.e. the rate of oxygen removal measured as percent per minute up to the 40% reduction level. The swelling property of the pellet is also very important in this section of the furnace.
At temperatures above 1000xc2x0 C., the pellet burden begins to soften slightly and at 1100xc2x0 C. molten slag begins to be produced and flows out with the rise in temperature. A standard reduction test under load (RTuL) is used to evaluate the pellet contraction at this temperature. At temperatures approaching 1200xc2x0 C., a cohesive layer is formed by the combination of metallic iron which grows in the shell portion, normally referred to as the Cohesive Zone. When the temperature exceeds 1200xc2x0 C. metallic iron and slag separate and in the vicinity of 1400-1500xc2x0 C. these begin to melt down (Melting Zone).
Higher grade iron ore pellets containing less than 2% silica can also be used as a feed stock for a coal or gas based direct reduced iron (DRI) process. The DRI processes normally operate at maximum temperatures of from 800 to 1100xc2x0 C. and the iron oxide pellets are reduced to metallic iron in the solid state below the softening/melting temperature of the iron oxide and gangue constituents in the pellet. The highly metallized pellets, i.e. 92+% metallization, are normally used as melting stock in combination with ferrous scrap for electric arc furnace steelmaking.
Iron ore pellets produced commercially owe their hardness to being fired or indurated at temperatures ranging from 1288 to 1343xc2x0 C. (2350 to 2450xc2x0 F.). This high temperature hardening process requires large quantities of heat energy and complicated and expensive processing equipment.
Under current practice, the high temperature firing of iron ore pellets is extremely demanding both technically and economically. The high temperature process requires large quantities of energy, ranging from 350,000 to 1,000,000 Btus and 35 kwh per long ton of pellets, depending on whether the iron oxide in the ore is in the form of magnetite or hematite. The pellet hardening operation consists of large, complicated furnaces, such as the grate-kiln, straight grate or shaft furnace to carry out the heat hardening on a continuous basis. Because of the high capital investment for a large pellet induration facility this approach is economically feasible for large scale operations exceeding one million tons of pellets per year, but not always feasible for smaller capacity operations.
Also a significant environmental problem associated with the high temperature induration process is the high thermal NOx emission. This is a serious concern for the iron ore producers and pollution control agency.
To overcome these problems and in an effort to reduce the high capital and energy operating costs associated with the conventional high temperature pellet induration process the invention provides a low-temperature hardening process for iron ore pellets that requires uncomplicated equipment and a minimum amount of energy. The green pellets produced by the process of the invention are dried and cured in a continuous drying oven at a maximum temperature of approximately 150xc2x0 C. (300xc2x0 F.). This treatment provides the pellets with surface properties that make them resistant to abrasion and weathering.
The prior art has shown use of binder formulations for agglomerating or pelletizing ore individually utilizing polyvinyl alcohol and silica. In particular, U.S. Pat. No. 3,661,555 to Kusama et al., U.S. Pat. No. 5,472,675 to Polizzotti et al. and U.S. Pat. No. 3,860,414 to Lang et al. disclose use of polyvinyl alcohol as a binder component in producing mineral pellets. U.S. Pat. No. 3,725,032 to Kihlstedt, U.S. Pat. No. 2,884,320 to Johnson and U.S. Pat. No. 4,985,075 to Ohno et al. disclose use of silica as an agglomerating agent in producing mineral pellets.
U.S. Pat. No. 2,833,661 to Iler discloses a formulation comprised of a colloidal silica and a polymer such as polyvinyl alcohol for use as a film-forming coating on paper substrates.
In contrast to the known prior art, the invention provides a binder formulation for use in agglomerating mineral concentrates comprised of both colloidal silica and polyvinyl alcohol. The binder formulation, in preferred applications, was developed for iron ore concentrate and fines suitable for both balling (green pellet formation) and low temperature hardening of the green pellets.
The binder formulation produces iron ore pellets with a bonding structure resistant to xe2x80x98room temperaturexe2x80x99 mechanical degradation as well as xe2x80x98heat loadsxe2x80x99 under reducing conditions so that the pellets do not decrepitate in subsequent high temperature reduction processes. The low temperature bonding process is an alternative to the high temperature hardening process currently in use in the U.S. and foreign countries for pelletizing iron ore concentrates and fines. The high temperature indurated pellets are used as feedstock for the blast furnace and other direct reduced iron (DRI) processes.
Thus it is a broad object of the invention to provide a binder formulation comprised of both colloidal silica and polyvinyl alcohol to produce hardened iron ore pellets from either magnetite or hematite concentrate or fines, or mixtures of both magnetite and hematite concentrates or fines.
Another object of the invention is to produce iron ore pellets from a low temperature bonding process that have mechanical and metallurgical properties suitable for the same use in ongoing iron and steel-making processes.
A specific object of the invention is to provide a low temperature hardening process that offers the advantages of lower capital costs and less environmental concern than conventional processes. Operating costs will depend on binder costs and requirements for different ore types.
Another specific object of the invention is to provide iron ore pellets produced from the low temperature process that have better reducibility properties than the conventionally made fired pellet because of their higher porosity after the organic binder burns off in the reduction process.
Another object of the invention is to provide a low-temperature process that permits the addition of a carbon source to the hardened pellets to accelerate the reducibility in subsequent iron making processes.
A further object of the invention is to use the binder formulation to agglomerate other mineral concentrates/fines, particularly those that can not be exposed to high temperatures, i.e. such as coal fines.
A further specific object of the invention is to provide iron pellets to be used as feed stock for an on-site coal or gas based direct reduced iron facility.
In the present invention, these purposes, as well as others which will be apparent, are achieved generally by providing a binder formulation for use in agglomerating mineral concentrates comprised of colloidal silica and a polymeric binder.
The colloidal silica is provided in the formulation by either a water dispersion of silica or in a powder form such as a clay-like mineral montmorillonite or bentonite. The preferred polymeric binder is a water-soluble polyvinyl alcohol. In the binder formulation the colloidal silica and polymeric binder are present in amounts sufficient to provide room temperature strength for transporting a pellet and high temperature strength for use in processes operating at high temperature.
The mineral concentrate of mineral fines are selected from the group consisting of magnetite, hematite and mixtures thereof. In addition, other mineral concentrates/fines, particularly those that can not be exposed to high temperatures, i.e. such as coal fines can be combined with the invention binder to provide desired strength properties.
The invention also provides a mineral pellet and related low temperature hardening process for making the pellet. The process involves mixing the binder formulation, comprised of colloidal silica and a polymeric binder, with the mineral concentrates or fines to be pelletized. Typically, the pellet comprises at least 96 dry wt. % or greater mineral concentrate or mineral fines, preferably between 97 to 98 dry wt. %; colloidal silica in an amount up to 2 dry wt. %, preferably 1 dry wt. %, and the polymeric binder in an amount up to 2 dry wt. %, preferably 1.5 dry wt. %. The pellet is dried at temperatures up to approximately 150xc2x0 C. (300xc2x0 F.) to form the hardened pellet.
The pellet formed by the invention process has sufficient room temperature strength to withstand transporting and has sufficient hot temperature strength for use in subsequent iron-making processes. Preferably, the pellets are used as a feed stock in a direct reduction furnace operating at temperatures up to 1100xc2x0 C. The pellet can also be used in blast furnace or other applications requiring agglomerated feed stocks.
Other objects, features and advantages of the present invention will be apparent when the detailed description of the preferred embodiments of the invention are considered with reference to the drawings, which should be construed in an illustrative and not limiting sense as follows: